In order to optimize the range, cost and ergonomics of such a system, it is desired to provide a continuous GC-PC zoom equipped with an optical TPC configuration, with a PC/TPC ratio of class 1.5, without the TPC configuration increasing the weight, bulk or cost of the system (GC being the acronym of the French expression grand champ meaning “large field”, PC being the acronym of the French expression petit champ meaning “small field” and TPC being the acronym of the French expression trés petit champ meaning “very small field”).
Continuous single-aperture IR zooms with no structural flux do currently exist.
Commercially available IR systems, such as for example the Sophie ZS or Sophie XF binoculars from THALES Optronics, are currently equipped with continuous GC-PC zooms of this type that have a an average ratio (typically of about 6). Conventionally, such a zoom, as shown in FIGS. 1A and 1B, comprises in order on the optical axis z of the system:
A fixed head group Gf1 of focal length F1, of telephoto type (a priori).
A divergent movable group Gm1 playing the role of variator, which works with a negative variable magnification gm1.
A convergent movable group Gm2 playing the role of compensator, which works with a variable magnification gm2 that is also negative.
A fixed group Gf2 that relays, to the detector, the real intermediate image delivered by the three groups Gf1, Gm1 and Gm2. The group Gf2 works with a constant and negative magnification gf2 that is generally comprised between −2 and −1.
A folding mirror 2 is placed in the group Gf2 in order to optimize bulk.
The exit pupil of the system coincides with the cold diaphragm 3 of the detector 1, which plays the role of aperture diaphragm of the zoom in the continuous GC-PC range. Using a field lens 4 generally present in the group Gf2, it is possible to constrain the position and the aberrations of the entrance pupil of the PC, in order to minimize the diameters of the components of the head group Gf1, which add significantly to the cost of the optic. In GC, most often the entrance pupil is virtual and thus the beams remain in the interior of the envelope of the useful PC beams.
The focal length F of the zoom is expressed by F=F1·gm1·gm2·gf2. The variation in focal length, in other words the zoom effect, is due to variations in the product gm1·gm2, which is minimum in GC and maximum in PC.
The assembly is optimized so that in PC, the absolute value of gm2 (also denoted |gm2|) remains lower than about 0.85. It is known that this condition makes it possible to use the compensator (Gm2) as focal group for the entirety of the continuous GC-PC range. Specifically, the axial sensitivity of the compensator Gm2, which relates the defocus of the image and the axial movement of the compensator, is expressed by (1−(gm2)2)(gf2)2.
Therefore, if |gm2| is sufficiently far from 1.0, then a small axial movement of the tandem induces a defocus of the image; in other words Gm2 made play the role of near focal group or may serve to compensate the thermal drifts of the combination.
Likewise, if the product gm1·gm2 remains below about 0.85 in PC configuration, then it is possible to use the tandem Gm1-Gm2 as focal group in the entirety of the GC-PC range.
Of course, with this type of optical architecture it is possible to obtain a continuous zoom of higher ratio, in order to cover a GC-TPC need. Grosso-modo, the diameter of the head components is then multiplied by a factor equal to the PC/TPC ratio, this having a substantial impact on the bulk and cost of the equipment.
Another known solution consists in using a bi-aperture GC-TPC infrared zoom with no structural flux. It has been seen that the elongation of the focal length of a zoom to obtain a TPC is naturally accompanied by an increase in the useful diameter of the head components, and therefore by an additional cost. Nevertheless, it is possible to limit the diameter of the frontal lens (head lens of the head group) while guaranteeing a perfectly healthy photometric behavior. To do this, the useful aperture in TPC is decreased by inserting in front of the cryostat a diaphragm boarded by a mirror for decreasing structural flux. Such a device is more precisely described in § 5 of the article by J. Vizgaitis: Dual F/number optics for 3rd generation FLIR systems, Proc. Of SPIE Vol. 5783. It is for example found in the ATTICA M-ER camera of the company AIRBUS Defense & Space, which is described in the article “New Thermal Imager for Long Range Surveillance” by J. Fritze & H. Schlemmer, AMA Conferences 2013. Such a solution solves only partially the stated problem in so far as it employs a mechanism dedicated to switching the TPC aperture diaphragm, this obviously not being very economical.
It is also possible to add an afocal TPC in front of a GC-PC zoom. Specifically, to obtain a TPC from a GC-PC zoom, it is enough to mount a PC/TPC magnifying afocal module in front of the head group of the zoom. On so doing, the GC is also decreased by a factor PC/TPC. Such an afocal system, of Galilean type, comprises at least two IR components of large diameter, which are a priori expensive. This solution absolutely does not solve the stated problem, neither in terms of weight and bulk, nor in terms of cost, nor in terms of ergonomics.
Therefore, there remains to this day a need for an IR imaging system that simultaneously meets all of the aforementioned requirements, in terms of weight, bulk, cost, and ergonomics.